1. Field of the Invention
The present invention relates to a process for working up liquid reaction mixtures such as are formed during dimethyl carbonate preparation by oxidative carbonylation of methanol in the presence of a copper-containing catalyst under increased pressure at elevated temperature, which is characterized in that the reaction solution obtained after removal of the catalyst is freed from the water of reaction by simple distillation under normal pressure, and the anhydrous top product, comprising methanol and dimethyl carbonate, is separated by distillation under increased pressure into pure dimethyl carbonate as the bottom product and a dimethyl carbonate-depleted mixture of methanol and dimethyl carbonate as the top product, from which the top product is recycled to the normal-pressure column or the reaction.
2. Description of the Related Art
In the past, a number of processes have been developed for the preparation of dialkyl carbonates by catalytic reaction of the starting materials methanol, carbon monoxide and oxygen.
In DE-A 2 110 194, metal complexes of groups IB, IIB and VIIIB of the Periodic Table, in particular the metals Cu, Ag, Au, Zn, Cd, Hg, Fe, Co and Ni, which can exist in two different oxidation levels in redox reactions, have been mentioned as suitable catalysts. This process gives good yields with complexed Cu.sub.2 Cl.sub.2, but has the disadvantage that the removal of the very expensive complex ligands and of the dissolved complexed catalyst from the reaction solution requires much effort.
In DE-PC 2 743 690, simple monovalent salts of copper are used as catalysts instead of the copper complex compounds. Although this process variant gives good yields of dialkyl carbonates, working-up of the reaction solution also presents major problems here, since the partly dissolved catalyst must be removed from the reaction solution. According to the doctrine of this patent, this is effected by filtering off of the suspended content and rectification or crystallization of the dissolved catalyst. Since the catalyst-containing reaction solutions carry the catalyst to other parts of the plant, a high expenditure on apparatus is necessary for working up the reaction solution and catalyst. Because of the corrosive properties, all apparatuses (tanks, pipelines, distillation, crystallization and filtration apparatuses) which come into contact with the catalyst must be made of corrosion-resistant material. The process thereby loses its attractiveness.
The same working-up problems mean that the use of synthesis gas instead of CO, as described in DE-C 3 045 767, must remain economically unattractive. The corrosion problems during working-up of the reaction solution caused by the copper-containing catalyst render processes which comprise further additions to the catalyst (for example EP-A 217 651, EP-B 090 977, U.S. Pat. No. 4,370,275) uneconomical.
An alternative in process technology for removal of the catalyst is disclosed in DE-A 3 926 709. In this process the copper-containing catalyst remains in the reactor. The dialkyl carbonate formed during the reaction is stripped from the reaction mixture by the reaction gas together with the water of reaction and methanol. This effect is in general achieved by passing through the reaction mixture a gas stream of 20 to 30 standard 1 of CO/O.sub.2 gas mixture per g of copper present as copper catalyst in the reactor. Disadvantages of this process are the very large amounts of gas which must be kept circulating, and the high energy costs thereby caused, as well as the problems in dispersion of the gas because of the large amounts of gas. In this procedure, the temperature and pressure of the reactor must additionally be regulated precisely in order to be able to maintain the level of liquid in the reactor, since even small variations in reactor temperature or pressure lead to the amounts discharged being changed significantly. The statements made in the case of DE-A 3 926 709 essentially apply to the processes described in EP 0 460 732 A1and EP 0 460 735 A2.
In all these processes, although the catalyst is removed more or less effectively, the process problems during working-up of the catalyst-free reaction solution also have not been solved in these Applications, that is to say removal of the water from the catalyst-free reaction solution and the separation of methanol and dimethyl carbonate (DMC), since these form an azeotrope.
According to DE-A 2450856, the removal of dimethyl carbonate from the water of reaction and methanol (MeOH) by means of a simple rectification is complicated, since various azeotropes form between DMC, MeOH and water. The doctrine of the patent application is separation by using extractive distillation with water as the solvent. However, the process is uneconomical, since considerable amounts of water are required for the separation (9.5 g of water per g of reaction solution).
DE-A 2607003 describes a separation experiment for splitting the methanol/dimethyl carbonate azeotrope by applying pressure and elevated temperature. According to the doctrine of this application, although a bottom fraction of pure DMC is obtained, the top product is again a mixture of methanol and DMC which is depleted in DMC. How such a process fragment can be integrated appropriately into a working-up process is not disclosed. In addition, the separation is carried out without the water unavoidably obtained during oxidative carbonylation, which, as disclosed in DE-A 3926709, forms a ternary azeotrope with methanol and dimethyl carbonate.
U.S. Pat. No. 4,162,200 also discloses a distillation by extraction where, for example, cyclohexane and chlorobenzene are employed as auxiliaries. Because of the use of reagents foreign to the system, additional distillation steps become necessary and these again impede an economic solution to the separation problem. In addition, these substances foreign to the system can lead to contamination of the dimethyl carbonate. A corrosive action is to be expected in particular if chlorobenzene is used, which leads to considerably higher costs.
U.S. Pat. No. 3,803,201 describes a process in which the methanol/DMC azeotrope is worked up by a combination of low temperature crystallization, filtration and subsequent fractional distillation. This process is unusable for an industrial application merely because of the temperature of about -70.degree. C. required. The use of crystallization, filtration and distillations furthermore represents a considerable expenditure on apparatus, which makes the process look uneconomical.
The object was to discover a simple and inexpensive process for the working-up of reaction solutions of dimethyl carbonate, methanol and water such as are formed during oxidative carbonylation of methanol using copper chloride as the catalyst.